Fix issues with the initial state when a SOS filter has no IIR part

[andfoy]

Fixes #7990

The issue presented by the OP raised some NaN values, which were caused when sosfilt_zi tried to solve the equation 0x = 0, which was due to a lack of IIR coefficients in the example bessel filter. This change is analogous to this call in lfilter:

cupy/cupyx/scipy/signal/_signaltools.py

Line 930 in ff9b39b

y_zi = cupy.nan_to_num(y_zi, nan=0, posinf=cupy.inf, neginf=-cupy.inf)

[takagi]

/test mini

[takagi]

LGTM!